Photosynthesis, is another form of carbon fixation that can take place in the absence of sunlight. The Calvin cycle has two parts. First carbon dioxide is ‘‘fixed’’. Then ATP and NADPH from the light reactions provide energy to combine the fixed carbons to make sugar. Animals and other organisms cannot take in CO2 directly. They have to eat other organisms or absorb other organic molecules in order to get the Carbon that you need. Autotrophs can build low-energy inorganic CO2 into high-energy organic molecules like glucose. Autotrophs are an organism that is capable of synthesizing its own food from inorganic substances using light or chemical energy such as green plants, algae and other bacteria. carbon fixation is the first step of a cycle
